In U.S. Pat. No. 3,369,634, assigned to the assignee hereof, there has been disclosed an energy absorbing device which is rather simple in construction and operation. This device has proven to be very acceptable and valuable since it operates on a simple principle which avoids the need for hydraulic fluid in valves, etc. which are difficult to adjust and, usually, allow the fluid to leak from them at points of wear, etc., sometime during their use. In general, those devices comprise two or more concentrically related tubes which are so arranged that the application of axial forces to the tubes cause them to move in telescopic relationship. Each tube is specifically constructed so as to form an annular space, within predetermined tolerance limits, with the adjacent tube.
A rather precisely sized helical coil may be located within the annular space so as to undergo continuous deformation between the adjacent walls of the tubular members. In general, when forces having axial components are initially applied to the prior art devices, energy will be absorbed with very small telescopic displacement until a predetermined amount of force is being applied to the device. Subsequently, as the force application continues, energy will be absorbed by larger distances of telescopic displacement, called "stroke. " The level of the initial force required to generate increased telescopic displacement may be controlled by varying the number of turns in the helical coil.
Since the development of the prior art device, it has become apparent that there are many instances in which it is desirable to prohibit telescopic motion in any appreciable amount unless a predetermined force is first applied to the energy absorbing device. Also, it is desired to be able to control such a device after that predetermined force is initially applied, and then continues to be applied, to the device. In other words, it has become necessary and desirable to be able to control the shape of a force-displacement curve in a manner which will allow the achievement of goals which may be specified by the desired functions. For example, if an elastic or semi-elastic structure is to be protected by the energy absorbing device, it may be desirable to minimize the dynamic overshoot which could occur in the protected member, thereby increasing the probability that the member may be subjected to acceleration and deceleration forces without damage or injury. It has become necessary, if such control is to be accomplished, to modify and improve upon the prior art structures if the advantages of simplicity, avoidance of hydraulic fluids, etc., are to be avoided.